This invention relates to a transparent resin material which enables the production of various transparent resin molded articles and which is preferably used in the production of video disk, compact disk, overwritable optical disk, storable, erasable and regeneratable optical disk, optical lens, etc.
In recent years, transparent resins have been used as a molding material for automobile parts, lighting appliances, electrical parts, sundry goods, etc. in which ordinary transparency is required. Further, the transparent resins have been applied as an optical material in which optical properties are important. Optical materials are required to have not only transparency but also other important properties which conventional transparent resins have been unable to satisfy.
As the material for optical disk substrate which is an example of optical materials, there are known a polycarbonate resin, a polymethyl methacrylate resin, a polycyclohexyl methacrylate resin, a copolymeric resin of an alkyl methacrylate and methyl methacrylate, styrene, or other monomer, a (co)polymer of a bulky ester group-containing methacrylic acid ester, a copolymer obtained by copolymerizing ethylene and a norbornene type hydrocarbon compound using a Ziegler-Natta catalyst [Japanese Patent Application Kokai (Laid-Open) No. 292,601/86], a (co)polymer which is a hydrogenation product of a (co)polymer obtained by subjecting a tetracyclododecene type hydrocarbon compound or a norbornene type hydrocarbon compound to ring opening (co)polymerization using a metathesis catalyst Japanese Patent Application Kokai (Laid-Open) No. 26,024/85] and a ring opening (co)polymer of a polar substituent-containing norbornene derivative [Japanese Patent Application Kokai (Laid-Open) Nos. 19,801/87 and 19,802/87].
The above transparent resin materials, however, do not satisfy all of the birefringenece, low hygroscopicity, mechanical strengths and adhesion to recording layer which are high requirements for a material for optical disk substrate.
For example, the polystyrene resin and the polycarbonate resin both having high birefringence give many errors during the regeneration of stored information by laser beam. The polymethyl methacrylate resin having high hygroscopicity is deformed owing to moisture absorption, to cause many errors during the regeneration of stored information and also is very likely to cause a property change of the recording film due to moisture absorption. The polycyclohexyl methacrylate resin has a low glass transition temperature, so it has low heat resistance. The copolymer of polycyclohexyl methacrylate and methyl methacrylate has high hygroscopicity. The copolymer of polycyclohexyl methacrylate and styrene has high birefringence and accordingly poor optical properties.
The (co)polymer of a norbornene type hydrocarbon obtained by polymerization using a Ziegler-Natta catalyst, a homopolymer of a tetracyclododecene type hydrocarbon compound obtained by metathesis ring opening polymerization or a hydrogenation product of the (co)polymer of norbornene type hydrocarbon has improved birefringence, hygroscopicity and heat resistance but has poor adhesion to recording layer because they have no polar group contributing to adhesion.
The optical material consisting of a ring opening (co)polymer of a polar substituent-containing norbornene derivative is improved in adhesion to recording layer because of the polar substituent but is unable to satisfy both high glass transition temperature and low water absorption because when a polar substituent giving a high transition temperature is selected the saturated water-absorption becomes high and when a polar substituent giving a low saturated water-absorption is selected the glass transition temperature becomes low. Further, this optical material has a problem that the durability for a long period is not sufficient because it contains unsaturated double bonds in the polymer structure.
Thus, substantially no satisfactory materials have existed as a transparent resin material which has sufficient optical properties, low hygroscopicity, good heat resistance and excellent adhesion to recording layer (i.e. durability).